The present invention relates to propulsion devices for ships and in particular to a water jet propulsion assembly comprising a propeller pump enclosed in a pump housing.
The pump wheel of the jet propulsion assembly of the kind concerned by this invention is normally mounted to the drive shaft which extends through the inlet canal. The drive shaft end is journalled in the pump housing by means of water-lubricated bushings, oil-lubricated spherical bearings or similar. See for instance the Swedish patent No. 8008288-6.
With a pump wheel mounted in such a way directly to the drive shaft, there is merely one problem present, i.e. the dimensioning of the drive shaft. Since the drive shaft disturbs the affluence of the water to the pump, the result is an uneven load on the pump wheel vanes. Such an uneven load induces a bending moment, which is transmitted to the pump shaft, and which has to be taken into consideration when dimensioning the shaft. It is appreciated that due to the bending moment the shaft be given a relatively generous diameter, which in turn increases the disturbance of the water flow to the pump, and thus renders increased losses. Another close problem consists in that the drive shaft deviates from the theoretically correct position, which leads to either increased wear on the bearing in the pump housing or, if the bearing is fitted to receive angular deviations, a large risk that the pump wheel vanes will bump into the inner walls of the pump housing, since the pump wheel is directly mounted to the drive shaft. For the best power economy possible, the pump wheel vanes should terminate as closely to the pump housing inner walls as possible.
The purpose of the present invention is to eliminate the above problems. The purpose is attained by the provision of a jet propulsion assembly of the above mentioned kind, wherein the drive shaft is fitted not to be affected by a bending moment or by other forces, for example pressure forces, from the pump wheel but only to transmit turning moments from the drive machinery, at the same time as the angular deflections of the drive shaft do not affect the precise bearing of the pump wheel. The actual invention object as well as that which is characteristic thereof are evident from the accompanied patent claims.
The invention will now be described in more detail, in conjunction with the attached drawings, which in the exemplary form of the invention show an embodiment of a jet propulsion assembly according thereto.